Dear all, I've found the solution.
The solution kindly provided by Michael didn't work because, as he wrote, that window is not a window application window with its own right.
The problem I had was caused by using two different screens with different solution. My laptop has some trouble when handling distance window in the second screen, when CAESAR II is maximized in the main window.
As he correctly wrote, the window was pushed outside the monitor's range.
Here it is the simple solution I found:
1) reduce the Caesar II window
2) Select the "select icon"
3) Edit > Distance

I had this problem by using CAESAR II 2018 on Windows 10.
One suggestion for the software developer: implement, for example in the Reset Toolbar Lauout, a command to recall the secondary window inside the area covered by the main CAESAR II window.
best regards